Re: [RFC][PATCH] fix move/migrate_pages() race on task struct

[Christoph Lameter]

On Thu, 23 Feb 2012, Dave Hansen wrote:

> > We may at this point be getting a reference to a task struct from another
> > process not only from the current process (where the above procedure is
> > valid). You rightly pointed out that the slab rcu free mechanism allows a
> > free and a reallocation within the RCU period.
>
> I didn't _mean_ to point that out, but I think I realize what you're
> talking about.  What we have before this patch is this:
>
>         rcu_read_lock();
>         task = pid ? find_task_by_vpid(pid) : current;

We take a refcount here on the mm ... See the code. We could simply take a
refcount on the task as well if this is considered safe enough. If we have
a refcount on the task then we do not need the refcount on the mm. Thats
was your approach...

>         rcu_read_unlock();

> > Is that a real difference or are you just playing with words?
>
> I think we're talking about two different things:
> 1. does RCU protect the pid->task lookup sufficiently?

I dont know

> 2. Can the task simply go away in the move/migrate_pages() calls?

The task may go away but we need the mm to stay for migration.
That is why a refcount is taken on the mm.

The bug in migrate_pages() is that we do a rcu_unlock and a rcu_lock. If
we drop those then we should be safe if the use of a task pointer within a
rcu section is safe without taking a refcount.

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